1. Field of the Invention
The present invention relates generally to a process which removes oxygen from biomass, and more particularly, not by way of limitation, to an improved hydrodeoxygenation process.
2. Brief Description of the Related Art
Biomass is a renewable alternative to fossil raw materials in production of liquid fuels and chemicals. Increase of biofuels production is part of the government's strategy to improve energy security and reduce green house gas emissions. However, most biomass has high oxygen content which lowers fuel quality and heat value. Upgrading biomass or biomass intermediates into high quality hydrocarbon fuels thus requires removal of oxygen. The biomass oxygen may be in the form of an ester, carboxylic acid or hydroxyl groups.
Removal of oxygen by catalytic reaction with hydrogen is referred to as hydrodeoxygenation (HDO). This reaction may be conducted with conventional fixed-bed bimetallic hydrotreating catalysts such as sulfided nickel-molybdenum (NiMo) or cobalt-molybdenum (CoMo) which are commonly used in refineries.
Unrefined vegetable oils and animal fats have several hundred ppm phosphorus in the form of phospholipids. In addition, animal fats may contain up to a thousand ppm metal chloride salts. The salts are soluble in the fat/grease feed, but come out of solution during the HDO reaction and plug the catalyst bed. The metals/salts can also deactivate the catalyst by reducing available pore surface. In the presence of free fatty acids, metal chlorides may form soluble soaps (e.g. calcium stearate). In such form, metals are difficult to remove using conventional cleanup technologies such as water washing.
Several prior art processes for producing fuels from starting materials such as plants and animals are known. Conversion of vegetable oils to n-paraffins has been reported in the prior art. Some prior art has shown that the process may be applied to other forms of biomass such as tall oil fatty acids, animal fats, and restaurant greases. Hydroisomerization of the bio-derived n-paraffins to isoparaffinic diesel has been taught in the prior art.
Other prior art describes use of feed treatment upstream of an HDO reactor. Overall cleanup efficiency of 75% is reported. However no attempt is made to improve the hydrocarbon yields by increasing HDO efficiency. In fact the CO+CO2 yields reported are as high as 15% (which is near theoretical maximum decarboxylation (DCO), suggesting very low HDO efficiency).
Still other prior art teaches use of feed dilution to achieve better catalyst life. Solvent dilution is used to lower average bed temperatures for the exothermic HDO reaction, which in turn reduces the heavy side products that are formed at high temperature and reduce catalyst activity. However the level of dilution reported for good results is between 3% and 20%. This level of feed dilution translates to a significant increase in reactor size and separation/recycle costs.
Use of pre-hydrogenation to saturate the double-bonds in the triglyceride is described in other prior art. However, this pre-hydrogenation describes the well-known vegetable oil hydrogenation process which does not remove any oxygen. As such, the hydrogenolysis load on the main HDO catalyst is not affected by such pre-hydrogenation.
To this end, although processes of the existing art utilize biomass to produce biofuels, further improvements are desirable to increase HDO efficiency, reduce water partial pressure in the reactor, achieve a low capital/low operating cost method of removing heat from the fixed-bed reactor system, or address the problems with high acid feeds (characteristic of fatty acid containing biomass), which impacts preheater/reactor metallurgy and catalyst attrition. It is to such a process that the present invention is directed.